This invention relates to methods and apparatus for testing optical fiber communication links in optical communication systems and, more particularly, to methods and apparatus for characterizing an optical fiber using optical amplifier light sources.
Optical transmission systems are quickly evolving to multi-Terabit per second (Tb/s) capacity. Successful deployment of these systems requires accurate characterization of the fibers that connect all the transmission equipment together. Such characterization information includes fiber attenuation, chromatic dispersion, polarization mode dispersion (PMD) and back-reflection among others. This information allows the network provider to deploy the appropriate compensation techniques to mitigate deficiencies in the fibers, and also allows the network provider to understand the ultimate transmission distance limitations within the system.
In the field, fiber characterization information must be evaluated in conjunction with the installation of new optical transmission systems or the upgrade of existing routes to higher bit rates. For example, control of the total chromatic dispersion of transmission paths is critical to the design and construction of long-haul, high-speed, high-capacity telecommunications systems. Similarly, PMD of older installed fibers is typically much higher than recently manufactured fibers, and system integrators often need to measure these installed fibers as they plan to upgrade their systems to higher bit rates.
Fiber characterization is of particular importance to high-capacity fiber optic systems such as Dense Wavelength Division Multiplexed (DWDM) systems with many high bit-rate (e.g. 10 Gb/s and above) channels in place. In many cases, it is not possible to install the system at all without this information. There are currently at least ten different optical fiber types examples of which include Non-Dispersion Shifted Fiber, Dispersion Shifted Fiber, True Wave Classic(copyright), True Wave Plus(copyright), True Wave Minus(copyright), True Wave RS(copyright), All Wave(copyright), LEAF(copyright), E-LEAF(copyright), LS(copyright), etc. Each of these fiber types have different characteristics yielding varying levels of performance. Identification of the fiber type is, therefore, essential in providing accurate characterization information as system upgrade costs differ radically based on the type of fiber installation.
Where the fiber characterization information may be estimated from knowing the fiber length and fiber type, it may be possible to deploy a system for applications that do not have extended reach or capacity. However, suppliers in this case would be unable to permit customers to deploy these systems in aggressive applications. For example, such a case may occur when a customer wants extended reach or capacity and is unwilling to characterize the fiber in full. In addition to the above, some customers do not have accurate records regarding the type of fiber they have installed in the field. This information is essential to allow them to estimate the fiber characteristics from the length.
Generally, the customer may estimate fiber characteristics from length and type information. However, this does not supply them with enough information for suppliers to specify the most aggressive system. Subsequently, the customer may have to take measurement equipment out to the field to collect accurate fiber characterization information. This requires a coordinated effort (potentially at both ends of the fiber) to identify the fiber strand in question, disconnect the transmission equipment, make the measurement, record the results, and communicate these results to the equipment deployment team.
Several measurement techniques presently exist for characterizing optical fibers, the most popular of which is Optical Time Domain Reflectometry (OTDR). Optical time domain reflectometers (OTDRs) are frequently used to measure a variety of optical fiber properties. OTDRs operate by sending a short pulse of laser light down an optical waveguide fiber and observing the small fraction of light that is scattered back towards the source. This small fraction of light represents attenuation and reflectance in the optical fiber under test. By measuring the amount of backscattered and/or reflected signal versus time, the loss versus distance of the optical fiber is measured. Furthermore, it is known that an OTDR can be employed in combination with a variable wavelength laser source in order to display the effect of wavelength dependent fiber attenuation and chromatic dispersion of an optical fiber path.
However, OTDR systems requiring a wavelength-tunable pulsed light source and/or multiple light sources for the test signals impose a high degree of complexity not to mention cost. Furthermore, the use of separate OTDR systems to collect reflectance data necessitates the dispatching of experienced craftspeople to the field with a high level of expertise to perform the measurements. An inexpensive technique for testing optical fibers without the use of a tunable OTDR pulsed light source would, therefore, be highly beneficial.
The present invention discloses a method for characterizing an optical transmission fiber using a modest enhancement to equipment that is ordinarily embedded within an optical transmission system. The apparatus of the invention is embedded in the transmission equipment itself, thereby eliminating the need to dispatch a craftsperson for performing field measurements. The method and apparatus of the present invention, therefore, eliminates the need for having to take the optical transmission system out of operation in order to isolate the optical transmission fiber to be characterized. Although it is already known to embed separate test equipment into a system application using established methods (e.g. OTDR), this is necessarily more expensive than the method and apparatus of the present invention since it requires the use of additional test laser sources.
According to a first broad aspect of the invention, there is provided a method for characterizing an optical transmission fiber comprising operating an optical light source so as to launch an optical signal into a first end of the optical transmission fiber, detecting at a second end of the optical transmission fiber a residual optical signal resulting from the optical signal launched into the first end of the optical fiber and characterizing the optical transmission fiber on the basis of the detected residual optical signal.
According to another broad aspect of the invention, a Raman pump laser is used as the light source for characterizing the optical transmission fiber.
According to another broad aspect of the invention, an optical service channel (OSC) laser source is used as the light source for characterizing the optical transmission fiber.
According to another broad aspect of the invention, a plurality of optical light sources, each operating at a particular wavelength, are used for characterizing the optical transmission fiber in a wavelength band defined by the plurality of optical light sources.
According to another broad aspect of the invention there is provided a method for characterizing an optical transmission fiber comprising operating an OSC laser source so as to launch an optical signal into a first end of the optical transmission fiber, detecting a reflected optical signal at the first end of the optical transmission fiber resulting from the optical signal launched into the first end of the optical transmission fiber and characterizing the optical transmission fiber on the basis of the reflected Raman pump light detected at the first end of the optical transmission fiber.
According to another broad aspect of the invention, there is provided an optical transmission system for transmitting multi-channel optical signals over an optical transmission path, the optical transmission path including an optical transmission fiber interposed between a first node and a second node wherein the multi-channel optical signals are transmitted over the optical transmission fiber from the first node to the second node, the optical transmission system comprising at least one optical light source provided at the second node operable to inject an optical signal into the optical transmission fiber for transmission to the first node, at least one photodetector at the first node for detecting a residual optical signal resulting from the optical signal injected into the optical transmission fiber at the second node and a processing agent for characterizing the optical transmission fiber on the basis of the optical signal injected at the second node and the residual optical signal detected at the first node.
According to another broad aspect of the invention there is provided apparatus for characterizing an optical transmission fiber comprising at least one optical light source operable to inject an optical signal into a first end of the optical transmission fiber, at least one photodetector at the first end of the optical transmission fiber for detecting a reflected optical signal resulting from the optical signal injected into the first end of the optical transmission fiber and a processing agent for characterizing the optical transmission fiber on the basis of the optical signal injected into the first end of the optical transmission fiber and the reflected optical signal detected at the first end of the optical transmission fiber.